Back in 4th Grade, Tanaka was schoolmates with Kanoka of Niiyama Girls' High. The latter was always bullied by boys due to her height while Tanaka would say she's awesome that way and would always cheer her up[1]. He introduced Kanoka to volleyball[1], which she struggled with at first but was still cheered on by Tanaka.
Tanaka later transferred to an unknown school in fourth grade and eventually advanced to Ōjitsu Junior High[3]. Despite both being from Miyagi Prefecture, the two only reunited at the Spring High National Tournament in Tokyo.
Back when Tanaka joined Karasuno, he had short, frizzy hair in a light color. He claimed to have fallen in love the very moment he laid eyes on Kiyoko and proposed to her right on the spot[5]. Kiyoko rejected him immediately, but that did not stop Tanaka from pursuing her. As a first year, Tanaka was much more volatile and impulsive. He couldn't stand the unfair treatment the volleyball club was getting from the school and the trash-talking by other teams. Tanaka originally wanted to retaliate but became more mature once he saw the determination and maturity of his upperclassmen. He shaved his hair the following day, claiming it was to help him feel refreshed.
Tanaka has a shaved head with a widow's peak and sharp blue-grey eyes (golden brown in the manga colored illustrations) which led many to mistake him as a delinquent. Back in his first year at Karasuno, Tanaka had short fuzzy hair dyed in a light color (blond in the anime). He is average in height and has a lean, but strong build. He tends to make intimidating faces to try to scare others, which Daichi scolds him for.
Post-timeskip, Tanaka appears to have a bit more of a muscular build that most likely resulted from his career and also has let his hair grow out a bit in its natural dark color.